The present invention relates to a CMP (Chemical Mechanical Polishing) method, particularly, to an abrasive composition used in the CMP method, a method of manufacturing said abrasive composition, and a method of polishing a semiconductor wafer using the abrasive composition.
A polishing apparatus comprises a polishing plate having a surface covered with a polishing pad and rotated by, for example, a motor and a suction plate for supporting the rotating polishing plate. In a general method of polishing a substrate using the polishing apparatus, a surface to be polished of the substrate, which is rotated, is pressed against the rotating polishing plate while supplying an abrasive material in the form of a slurry to the polishing point. The polishing technology utilizing the polishing apparatus is employed in the manufacture of, for example, a semiconductor device.
The manufacturing method a semiconductor apparatus such as an IC or an LSI comprises a design step for designing an integrated circuit which is to be formed on a semiconductor substrate, a mask preparation step for depicting electron beams used for forming the integrated circuit, a wafer manufacturing step for forming a wafer having a predetermined thickness from a single crystal ingot, a wafer processing step for forming a semiconductor device such as an integrated circuit on the wafer, an assembling step for separating the wafer into individual semiconductor devices and for packaging the separated semiconductor device to obtain a semiconductor apparatus, and inspecting the resultant semiconductor apparatus. It is necessary to use a special manufacturing apparatus in each of these process steps.
An etch back RIE (Reactive Ion Etching) is known as a typical method employed in the wafer processing step for burying an optional material such as a metal, polycrystalline silicon (polysilicon), or a silicon oxide film (SiO.sub.2) in a concavity such as a trench or a contact hole, followed by flattening the surface thereof.
However, the etch back RIE method leaves much room for further improvement. For example, the etch back RIE method requires many process steps including the step of coating an etch back resist. Also, the wafer surface tends to receive a RIE damage, making it difficult to flatten the wafer surface sufficiently. Further, since a vacuum-based apparatus is used, the apparatus used is rendered complex in construction. Still further, a dangerous etching gas is used in the etch back RIE method.
Under the circumstances, the CMP method referred to previously attracts attentions recently in place of the etch back RIE method.
Where a polishing apparatus is used for flattening a silicon oxide (CVD SiO.sub.2) film buried in a concavity of a semiconductor substrate by a chemical vapor deposition or a silicon oxide (CVD SiO.sub.2) film formed by a chemical vapor deposition for use as an interlayer insulating film in a multi-layer wiring, an over-polishing brings about a dishing problem, i.e., the phenomenon that the over-polished region is recessed in the shape of a dish. In order to prevent the dishing problem and to stop the polishing at a desired film thickness, a stopper film is generally used in the polishing apparatus.
Where, for example, an oxide film is polished, used is a slurry-like abrasive material prepared by dispersing cerium oxide particles or silica particles as abrasive particles. The abrasive material having silica particles dispersed therein exhibits a low polishing rate. On the other hand, the abrasive material having cerium oxide particles dispersed therein exhibits a high polishing rate.
However, where an abrasive slurry having cerium oxide particles dispersed therein is used as an abrasive material and a silicon nitride film is used as a stopper film, a ratio of the silicon oxide film polishing rate to the silicon nitride film polishing rate, i.e., a silicon oxide film/silicon nitride film selectivity ratio, is about 2. On the other hand, where a polysilicon film is used as a stopper film, the silicon oxide film/polysilicon film selectivity ratio is as low as about 1 to 2. It follows that an over-polishing is brought about so as to remove partly even the stopper film.
Also, where an abrasive slurry having silica particles dispersed therein is used as an abrasive material and a silicon nitride film is used as a stopper film, the silicon oxide film/silicon nitride film selectivity ratio is 2. Even in the case of using a polysilicon film as a stopper film, the silicon oxide film/polysilicon film selectivity ratio is as low as 1. It follows that an over-polishing is brought about to remove partly the stopper film even in the case of using silica particles as abrasive particles.
As described above, an abrasive material exhibiting a sufficiently high polishing rate was not used in the conventional CMP method. Even if the abrasive material exhibits a relatively high polishing rate, the oxide film/stopper film selectivity ratio was low, making it difficult to prevent completely the dishing problem. In addition, a process margin was small.
It was customary in the past to use a thick stopper film in order to prevent the dishing problem caused by the low selectivity ratio, i.e., a ratio in polishing rate of the film to be polished to the stopper film. However, it is necessary to make the stopper film as thin as possible in view of the miniaturization of the apparatus, the polishing efficiency, etc.
Under the circumstances, an improved abrasive slurry is proposed in, for example, Japanese Patent Application No. 8-110575. It is proposed that an abrasive slurry prepared by dispersing abrasive particles consisting essentially of silicon nitride (Si.sub.3 N.sub.4) in a pure water or an ultra pure water be used in the CMP method. Where a silicon nitride film is used as a stopper film, the abrasive slurry disclosed in JP '575 noted above exhibits a silicon oxide film/silicon nitride film selectivity ratio of 10 to 20. Therefore, compared with the abrasive slurry containing cerium oxide particles or silica particles as the abrasive particles, the abrasive slurry containing silicon nitride particles as the abrasive particles is effective when used in the process employing a silicon nitride film as a stopper film. Further, the abrasive slurry containing silicon nitride particles as the abrasive particles provides a large margin relative to the over-polishing so as to diminish the dishing problem.
Even under the above-noted selectivity ratio, however, it is impossible to prevent completely the dishing problem, making it important to develop an abrasive material exhibiting a higher selectivity ratio. It should also be noted that silicon nitride particles tend to be precipitated and agglomerated, with the result that, where the abrasive slurry is stored over a long period of time, the silicon nitride particles are solidified at the bottom of the slurry container. In this case, it is difficult to disperse again the solidified silicon nitride particles.
Under the circumstances, it is difficult to employ the conventional CMP method in the mass production process of semiconductor devices.